For more than a century, the petrochemical industry has been developing efficient methods to convert readily available hydrocarbons (oil/petroleum distillates) to oxidized molecules of interest for use as commodity chemicals, polymer precursors, plasticizers, paints, coatings, fine chemicals, and pharmaceutical synthons. As the price of oil and the environmental risks posed by its use continue to rise, a significant amount of research is now being directed toward the reverse process of how to efficiently deoxygenate complex feedstocks to generate reduced hydrocarbons.
One desirable example of this is the synthesis of full-performance renewable fuels. Cyclic terpenes are compelling feedstocks for the generation of such high performance renewable fuels; their ring structures result in higher densities and volumetric net heats of combustion compared to linear or branched-chain alkanes. Important monoterpenes for use as fuels are pure hydrocarbons such as α-pinene or limonene, but oxygenated terpenoids including ethers and alcohols may also be considered important fuel precursors. For example, 1,8-cineole, which comprises ˜90% of eucalyptus oil, is a terpenoid ether that has been studied as a precursor to the bioaromatic compound p-cymene. Currently about 7,000 metric tons of 1,8-cineole are produced annually, but the potential worldwide annual production has been estimated at several million metric tons, with Australian capacity alone estimated at 800,000 metric tons. In addition to naturally occurring eucalyptol, 1,8-cineole and other terpenes are biosynthetic targets and a recent report has shown that 1,8-cineole can be generated from biomass sources by the fungus Hypoxylon sp. The use of fungi and other organisms that can convert crude biomass sources into terpenoids like 1,8-cineole has the potential to greatly reduce the cost, and increase the availability, of these renewable hydrocarbons.
Along with 1,8-cineole, a host of oxygenated terpenoids can be obtained from plant distillates. For example, hydrodistilled pine oil from Pinus armandii in Southwest China is composed of 33% oxygenated terpenoids. Solvent grade gum spirit turpentine can contain roughly 10% of a mixture of 1,4- and 1,8-cineole. Crude sulfate turpentine (CST)—which represents the bulk of commercial terpene production—typically contains small but important amounts of oxygenated terpenoids. For example, CST produced in the southern United States contains about 3-7% oxygenated terpenoids, while western mills generate CST with 8-20% oxygenated hydrocarbons. In contrast, other low-grade turpentines produced by the paper industry and called “red oils” are highly oxygenated due to treatment with aqueous acid solutions to promote distillation of mercaptans and recovery of methanol. This red oil is currently considered a waste product and is typically disposed of by incineration.
The conversion of 1,8-cineole to the bioaromatic p-cymene with transition metal doped gamma alumina was recently described, as was the isomerization of α-terpineol to 1,4- and 1,8-cineole with H3PW12O40. Homogenous systems have also been explored. For example, the hydration of α-pinene and CST with dilute sulfuric acid to generate α-terpineol and other terpene isomers has been studied. The reverse reaction, dehydration of α-terpineol with aqueous oxalic acid, was examined in 1961.
Although oxygenated terpenes such as cineoles and α-terpineol are desirable for fragrance, flavor, and antiseptic applications, the complicated distribution of products in crude feedstocks such as red turpentine oil make these mixtures much less valuable for specialty applications and therefore more intriguing for the production of renewable fuels
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.